A suicide bomber killed at least 22 people and wounded another 60 when he hurled himself at policemen standing outside the high court in the eastern Pakistani city of Lahore this morning.

The attack comes at a time of growing crisis in Pakistan, five weeks before contentious general elections and amid soaring flour prices.

Security forces were already on high alert for the start tomorrow of Muhurram, a major Shia religious festival that has traditionally seen sectarian violence.

The suicide bombing was the largest against civilians since the December 27 assassination of opposition leader Benazir Bhutto, which killed another 20 people and plunged the country deep into political chaos.

The interior ministry declared a heightened national security alert in response to today's attack, which took place as lawyers were meeting inside the high court precinct in preparation for their weekly anti-government demonstration.

Several senior judges and lawyers' leaders have been under arrest since early November, and previous protests by lawyers have seen violent street clashes.

Police officials said that just before midday a man leaped off a motorcycle, rushed towards a line of police outside the court, and blew himself up.

The city's police chief, Malik Muhammad Iqbal, said all but one of the victims were policemen.

"It appears the bomber was on foot and, as soon as some policemen tried to stop him, he blew himself up," he told Geo television.

Amid the carnage a photographer with the Associated Press said he saw the severed head of a man with long hair and a beard, possibly that of the bomber.

Outside the courtroom there was an outpouring of anger at the government for failing to prevent the carnage.

"How do they come to know [suicide bombers] have entered one of the provinces? What is the source? Can't the source be used to catch one of them?" one angered lawyer told Dawn television.

As is usual in Pakistan no group claimed responsibility, although President Pervez Musharraf has blamed Taliban commander Baitullah Masood for the Bhutto killing.

Her supporters say they suspect official involvement in the assassination.

The murky links between jihadist groups and elements of Pakistan's security forces were highlighted earlier this week with the arrest of a retired army officer accused of plotting a suicide attack on an air force bus in Sargodha on November 1.

Local media reports linked retired Major Ahsanul Haq to sectarian jihadist groups that previously fought proxy wars on behalf of the Pakistani military in Afghanistan and Indian-occupied Kashmir.

Today's attack increases the sense of destabilisation in Pakistan. Until now, Lahore, the capital of the politically influential Punjab province, had escaped the wave of suicide violence that has rocked northern cities such as Islamabad and Peshawar.

And while election campaigning has been marred by allegations of government-sponsored manipulation, the sense of discontent has been heightened by widespread electricity rationing, gas shortages and high flour prices.